Posttranslational modifications of proteins play a critical role in the regulation of signal transduction from receptors, chromatin remodelling and gene transcription. These modifications include acetylation, methylation, phosphorylation, ubiquitinylation, SUMOylation. EZH (enhancer of zeste homolog) 1 and 2 are the catalytic subunits of the Polycomb Repressor Complex 2 (PRC2) and exhibit methyltransferase activity that can catalyse the methylation of lysine amino acids (Margueron R, Reinberg D: The Polycomb complex PRC2 and its mark in life. Nature. 2011 Jan. 20; 469 (7330):343-9)
EZH1 and EZH2 play a critical role in the epigenetic long term silencing of gene expression by di- or tri-methylating lysine 27 of histone H3 (H3K27me2/3). Histone H3 is one of the five main histone proteins involved in the structure of chromatin in eukaryotic cells. Chromatin is the complex combination of DNA and protein that makes up chromosomes. It is found inside the nuclei of eukaryotic cells and is divided between heterochromatin (condensed) and euchromatin (extended). The basic building blocks of chromatin are nucleosomes, each of which is composed of 146 base pairs of DNA wrapped around a histone octamer that consists of 2 copies of each H2A, H2B, H3 and H4. The functions of chromatin are to package DNA into a smaller volume to fit in the cell, to strengthen the DNA to allow mitosis and meiosis, and to serve as a mechanism to control gene expression and DNA replication. The chromatin structure is controlled by a series of post translational modifications to histone proteins, notably histones H3 and H4, and most commonly within the “histone tails” which extend beyond the core nucleosome structure. Binding of enzymes and adaptor proteins to posttranslational modification in histone tails regulates chromatin dynamics and gene expression. H3K27me3 is thought to silence gene expression by recruiting histone deacetylases to the modified nucleosomes and stall transcriptional elongation by polymerase II. Thus, inhibition of the enzymatic activity of EZH1 and EZH2 may result in a loss of H3K27me3 and up-regulation of target genes.
In addition to its nuclear function in histone H3 modification EZH2 has been implicated in the regulation of signal transduction that leads to actin polymerization in the cytoplasm of cells (Su I H, Dobenecker M W, Dickinson E, Oser M, Basavaraj A, Marqueron R, Viale A, Reinberg D, Wülfing C, Tarakhovsky A: Polycomb group protein ezh2 controls actin polymerization and cell signaling. Cell. 2005 May 6; 121(3):425-36). The reorganization of the actin cytoskeleton critically contributes to T cell responses by facilitating the interaction of T cells with antigen presenting cells or target cells. In addition, actin remodelling plays an important role in T cell migration and motility during their recruitment to the sites of inflammation. A fraction of EZH2 protein was found to localize to the cytoplasm of T cells and to interact with the small GTPase VAV1, which is involved in actin remodelling. Genetic elimination of EZH2 resulted in impaired polymerization of actin in TCR stimulated T cells or at the T cell-antigen presenting cell interphase. Furthermore, actin polymerization induced by EZH2 over-expression was dependent on the methyltransferase activity of EZH2. Proliferation of T cells in response to TCR was also impaired in the absence of EZH2. Thus, inhibition of EZH1 and/or EZH2 may suppress the activation of T cells.
Mature T cell respond to foreign peptide antigens in the presence of appropriate co-stimulation by antigen presenting cells. They have the capability to discriminate between self and non self as a consequence of the selection of a TCR repertoire specific for foreign antigens in the thymus, tolerance induction of self reactive T cell clones in the periphery, and control of T cell activation by self antigen by regulatory T cells. T cells provide protection against different classes of pathogens by mediating distinct types of adaptive immune responses as a consequence of the expression of distinct sets of cytokines and other soluble and cell-bound products. In addition, they act as principle amplifiers and inducers of the appropriate inflammatory and effector responses in cells of the innate immune system and nonimmune cells. While such concerted immune responses can provide powerful protection against pathogens it can also result in inflammation associated with unwanted immune responses against self and environmental antigens and commensal microorganisms as well as collateral damage to the host as a side effect of immune responses against pathogens. CD8 T cells can lyse cells bearing intracellular pathogens but may also contribute to tissue damage and secrete pro-inflammatory cytokines, e.g. TNF and IFNg. CD4 T cells can have diverse functions in inflammation depending on their specific cytokine expression profiles. CD4+ Th1 cells are important for the clearance of intracellular pathogens but also play a critical role in inflammation through the expression of TNF and IFNg. IL-17 expressing CD4+ Th17 cells, which mediate neutrophilia and tissue remodelling and repair, have also been shown to be involved in many inflammatory conditions. CD4+ Th2 cells are involved in allergic responses by expressing IL-13, IL-5 and IL-4 which mediate airway hyper reactivity, eosinophil recruitment and IgE production. Thus, T cell activation is considered central to many inflammatory immune diseases. Accordingly, compounds that inhibit EZH1 and/or EZH2 activity and suppress T cell activation would be useful for the treatment of T cell mediated inflammatory immune diseases.
Inhibitors of EZH1/EZH2 that are useful in treating cancer have been reported in PCT applications PCT/US2011/035336, PCT/US2011/035340, and PCT/US2011/035344.